Cryocoolers are often used for cryogenic cooling of large superconducting magnet systems, used for MRI, NMR, research or large-scale industrial applications like magnetic separation. They are used either as shield cooling devices to reduce heating of the magnet which can be contained in a liquid, typically helium, or in a vacuum. Pulse Tube Refrigerators (PTR""s) have recently become commercially available with cooling powers in the required range for the applications mentioned above. These cryocoolers can now be considered for use in these systems. A PTR is a type of cryocooler which has no moving cold parts. Potentially this type of cryocooler offers lower service costs and significantly lower vibration signatures than other commercial alternatives like Gifford McMahon (GM), GM/Joule Thompson (JT) or Stirling cycle cryocoolers.
When cryocoolers of any type are used to cool large superconducting magnet systems for the applications described above, they are required to be extremely reliable and be serviced with the minimum interruption to the application or process. One factor affecting the long term reliability of all cryocoolers is the purity of the working refrigerant fluid, in this case helium gas. During the service operation the cryocooler system has to be opened up to replace serviceable parts and there must be no ingress into the system of any contaminant gas including, for example, air. In ideal conditions the PTR would be serviced with the cryocooler cold stages at cryogenic temperature. Thus if the part containing the cold stages is opened to air in the service operation, air and other contaminants will cryopump and become trapped onto the cold stages inside the machine. Normally this makes the PTR inoperable without warming the cold parts to room temperature and purging the air from the system with helium gas.
In accordance with the present invention, a pulse tube refrigerator comprises a fixed pressure casing containing cold parts and a removable pressure casing containing serviceable parts; wherein the fixed pressure casing and the removable pressure casing are coupled together via a joining member; wherein during cooling operation the joining member is arranged such that refrigerant fluid flows between the fixed and the removable casings; wherein during servicing the joining member is arranged to cut off flow of refrigerant fluid between the fixed and removable casings, such that refrigerant in the fixed casing is trapped and parts in the removable casing are accessible for servicing; wherein after servicing, substantially pure refrigerant fluid is pumped into the removable casing; the fixed and removable casings are re-joined and the joining member is arranged such that refrigerant fluid flows between the casings again.
The present invention allows a cryocooler system incorporating a pulse tube refrigerator to be opened up and serviced such that no air or contaminant gasses are admitted to the cold parts of the system. The cold parts are kept at cryogenic temperature without affecting the future performance of the system by degrading the purity of the refrigerant fluid.
The removable casing could be directly connected to the fixed casing, with the joining member comprising a seal between them, but preferably, the joining member comprises a clamp section and a seal.
This has the advantage that the clamp section can be allowed to move far enough from the fixed casing to allow the seal to be correctly positioned to cut of fluid flow, without opening up the serviceable parts until the cold parts are properly sealed off.
Preferably, the seal comprises a rotatable disc.
Provided that the disc is able to seal off the flow passages for closure, the size of the apertures is not constrained, but preferably, the rotatable disc is provided with apertures having substantially the same cross section as that of flow passages between the fixed and removable casings.
Preferably, the clamp section is provided with clamping means adapted to limit the extent of movement of the joining member away from the fixed casing.
This reduces the likelihood of contaminated gas entering the cold parts.
Preferably the removable casing is detached from the clamp section to allow access for servicing.